Fire Season, Overstory Density and Groundcover Composition Affect Understory Hardwood Sprout Demography in Longleaf Pine Woodlands

Abstract

Seasonal timing of prescribed fire and alterations to the structure and composition of fuels in savannas and woodlands can release understory hardwoods, potentially resulting in a global increase of closed-canopy forest and a loss of biodiversity. We hypothesized that growing-season fire, high overstory density, and wiregrass presence in longleaf pine woodlands would reduce the number and stature of understory hardwoods, and that because evergreen hardwoods retain live leaves, dormant-season fire would reduce performance and survival of evergreen more than deciduous hardwoods. Understory hardwood survival and height were monitored over seven years in longleaf pine woodlands in southwest Georgia with a range of overstory density, groundcover composition, and season of application of prescribed fire. Hardwood stem survival decreased with increasing overstory density, and deciduous hardwoods were more abundant in the absence of wiregrass. Contrary to expectations, evergreen hardwood growth increased following dormant-season fire. Differences in hardwood stem survival and height suggest that low fire intensity in areas with low overstory density increase the risk that hardwoods will grow out of the understory. These results indicate a need for focused research into the effects of groundcover composition on hardwood stem dynamics and emphasize that adequate overstory density is important in longleaf ecosystem management.

Longleaf pine proximity effects on air temperatures and hardwood top-kill from prescribed fire

“…For example, at an NDI of 0.1 (equivalent to 3 m distance from a 45 cm dbh longleaf pine), top-kill probability of mesic oaks and fleshy-fruited trees was 97%, compared to 88% for upland oaks and xeric oaks. This finding is consistent with that of Whelan et al (2018), who also found that top-kill probability of these groups becomes clearly differentiated when hardwood height is >1.5 m. It appears that, when hardwoods are small (~1 m height), fire from surface fuels alone is enough to top-kill them, and the fuel from pine trees is in excess of the requirement for effective top-kill. When hardwoods are larger and more fire tolerant, it takes the additional heating duration provided by fine needle fuels to top-kill them.…”

“…Most trees were second-growth longleaf pine that recruited to the canopy after timber harvest early in the twentieth century (Pederson et al 2008). An understory of hardwood sprouts <2 m height was present as clusters emanating from rootstocks whose density was estimated at 1000 to 2000 ha −1 (Whelan et al 2018). The ground cover was dominated by wiregrass (Aristida stricta Michx.)…”

“…Treatments include the following silvicultural treatments: g = group selection, r = group selection with reserves, s = single-tree selection, c = control. Time = period when fire burned, RH = relative humidity, T = air temperature, Gust = maximum wind speed recorded per 3 s recording intervals, ROS = rate of spread of fire, and P = number of days since last precipitation (Whelan et al 2018), so we were able to identify stems that originated before the prescribed fire. In each 2 m radius plot, we located clumps of stems from 0.5 to 5 m tall presumed to have emerged from a common rootstock (genet) prior to the fire; identified the species and counted the stems; and visually estimated height of the tallest stem to the nearest 0.33 m. If fire had burned through the 2 m radius plot, we visually estimated scorch height (via leaf discoloration) on the tallest stem per genet.…”

“…Once xeric oaks are 2 m in height, they have become extremely resilient to fire. Although evergreen oaks may grow more rapidly in height than the deciduous oaks (Whelan et al 2018), the rapid height growth may not be enough to compensate for the thin bark and high top-kill probability.…”

“…Fire is particularly important for regulating the dominance of oaks (Quercus spp. L.) and other broadleaved resprouting trees (henceforth, hardwoods), which often are present in high numbers in the understory of these forests (Provencher et al 2001;Whelan et al 2018). Pines provide fuel for prescribed fire through their shed needles, and careful selection of trees for harvest to maintain an evenly distributed rain of needles is a central element of ecological silviculture (Mitchell et al 2009;Neel et al 2010).…”